VR HMD eye box and sweet spot

• "sweet spot" is central Field of View region with good resolution and minimal artifacts
• "eye box" is 3D space in which eye pupil can enjoy "sweet spot".
    eye box combines eye relief and exit pupil

Eyebox:  3D tolerances for pupil location

Sweet spot:  ideally, field-of-view for resolution limited by pixel size

Pancake lens - expanded view